The National Grid <ul><li>Lesson Objectives: </li></ul><ul><li>To know why we need the National Grid. </li></ul><ul><li>To know how electricity is transferred via the National Grid </li></ul>KEYWORDS: NATIONAL GRID, TRANSFORMER, VOLTAGE
How Does Electricity Reach Us? <ul><li>Electricity reaches out homes by the National Grid. </li></ul><ul><li>The national grid is a network of cables which connect power stations to our homes and other buildings. </li></ul><ul><li>The network also contains TRANSFORMERS . </li></ul><ul><li>Two types of transformer are used; step-up transformers and step-down transformers. </li></ul>04/04/10
How Does Electricity Reach Us? Power is generated at power stations at around 25kV. It passes to a step-up transformer where it is transformed to 132kV. The electricity is transferred via thick, insulated cables, (aluminium covered with a ceramic insulator) held up by pylons to electricity sub-stations which are step-down transformers. At the sub-station, the 132kV voltage is transformed again to the 240V which we use in our homes. From the sub-station, the electricity travels via underground cables rather than overhead cables. 04/04/10
Why Do We Use Transformers? Why is electricity not transferred through the national grid at the 240V we use in our homes? Efficiency! Less energy is lost when transferring electricity at high voltages. In fact, by making the grid voltage 132kV or higher, the energy losses are reduced to almost zero! How is the energy lost at low-voltages? Why do we need to convert the voltage down to 240V?
The National Grid If electricity companies transmitted electricity at 240 volts through overhead power lines there would be too much ______ loss by the time electricity reaches our homes. This is because the current is ___. To overcome this they use devices called transformers to “step up” the voltage onto the power lines. They then “____ ____” the voltage at the end of the power lines before it reaches our homes. This way the voltage is _____ and the current and power loss are both ____. Words – step down, high, power, low, high
Transformers Transformers are used to _____ __ or step down _______. They only work on AC because an ________ current in the primary coil causes a constantly alternating _______ ______. This will “_____” an alternating current in the secondary coil. Words – alternating, magnetic field, induce, step up, voltage We can work out how much a transformer will step up or step down a voltage: Voltage across primary (V p ) No. of turns on secondary (N s ) Voltage across secondary (V s ) No. of turns on primary (N p )
Some transformer questions 04/04/10 Primary voltage V p Secondary voltage V s No. of turns on primary N p No. of turns on secondary N s Step up or step down? 12V 24V 100 ? ? 400V 200V 20 ? ? 25,000V 50,000V 1,000 ? ? 23V 230V 150 ? ?
Underground or Overground? <ul><li>What do you think of electricity pylons? </li></ul><ul><li>Think of some reasons why people object to them. </li></ul><ul><li>Objections to Pylons: </li></ul><ul><li>They spoil the landscape – pylons in fields </li></ul><ul><li>The high electric currents produce electric and magnetic fields which may affect people’s health - Electro-Sensitivity!!! </li></ul>
Underground or Overground? <ul><li>So, why don’t we bury all of the bacles underground? </li></ul><ul><li>It’s more expensive than using pylons </li></ul><ul><li>It would be much more difficult to repair – think of the problems we have when a water main bursts! </li></ul><ul><li>They would be difficult to bury – how thousands of miles of cable cover fields, rivers, canals and roads. </li></ul><ul><li>Also, think about the effect on people’s health – overhead cables are 50-60m high where as underground cables would be a few metres deep! </li></ul>
Further Questions 04/04/10 <ul><li>A transformer increases voltage from 10V to 30V. What is the ratio of the number of turns on the primary coil to the number of turns on the secondary coil? </li></ul><ul><li>A step-down transformer has twice as many turns on the primary coil than on the secondary coil. What will be the output (secondary) voltage if the input voltage is 50V? </li></ul>Primary voltage V p Secondary voltage V s No. of turns on primary N p No. of turns on secondary N s Step up or step down? 6V 24V 100 ? ? 400,000V 200V ? 1,000 ? 25,000V ? 20,000 20 ? ? 230V 150 1,500 ?